1. Field of the Invention
The present invention relates to a voltage-controlled oscillator (VCO) using a piezoelectric vibrator, and more particularly to a voltage-controlled oscillator which uses a quartz-crystal element as a piezoelectric vibrator and a transistor as an element for oscillation and amplification, achieves a large amount of change in frequency, and allows a reduction in circuit size.
2. Description of the Prior Art
A voltage-controlled oscillator using a piezoelectric vibrator such as a quartz-crystal element and providing an oscillating frequency depending on an applied control voltage is incorporated in, for example, a PLL (Phase Locked Loop) circuit. In recent years, it is desired that the voltage-controlled oscillator is reduced in size by forming circuit elements except a quartz-crystal element as an IC (Integrated Circuit) and exhibits a large change in capacitance with respect to the applied control-voltage, that is, a large change in the oscillating frequency.
FIG. 1 shows an exemplary configuration of a conventional voltage-controlled oscillator. The voltage-controlled oscillator comprises bipolar transistor 1 for oscillation, quartz-crystal element 2 as a piezoelectric vibrator, and two variable capacitance diodes D1, D2. Transistor 1 has a grounded emitter and a collector connected to power supply Vcc through load resistor 4. Quartz-crystal element 2 is connected between a base and the collector of transistor 1. A connecting point of quartz-crystal element 2 and transistor 1 is referred to as node A.
First variable capacitance diode D1 is inserted between node A and a ground point through first capacitor C1 for DC blocking. Second variable capacitance diode D2 is inserted between the collector of transistor 1 and a ground point through second capacitor C2 for DC blocking. The voltage-controlled oscillator is provided with control voltage Vc from the outside to change the capacitance values of variable capacitance diodes D1, D2. Control voltage Vc is applied to a connecting point (node B) of capacitor C1 and first variable capacitance diode D1 through resistor 3a for radio frequency blocking, and to a connecting point (node C) of capacitor C2 and second variable capacitance diode D2 through resistor 3b for radio frequency blocking. In addition, resistor 5 for self-bias is inserted between the collector and the base of transistor 1 to supply a base bias voltage from power supply Vcc to transistor 1.
The portion of the voltage-controlled oscillator except quartz-crystal element 2 is formed as an integrated circuit and incorporated into an IC chip.
In the voltage-controlled oscillator, quartz-crystal element 2 and variable capacitance diodes D1, D2 constitute a resonant circuit or a resonant loop. Specifically, a resonant circuit including quartz-crystal element 2 as an inductance component and variable capacitance diodes D1, D2 as capacitance components is formed. The resonance frequency component of the resonant circuit is fed back to and amplified by transistor 1 to obtain oscillation output Vout from the collector side.
While the oscillating frequency of the oscillator generally depends on the resonance frequency of the resonant circuit, the capacitance of transistor 1 or the like is added in. As a result, the oscillating frequency is determined by the series equivalent capacitance on the circuit side viewed from both ends of quartz-crystal element 2, that is, the load capacitance of quartz-crystal element 2. Since control voltage Vc applied from the outside changes the capacitance values of variable capacitance diodes D1, D2, the load capacitance of quartz-crystal element 2 is also changed. Thus, the oscillating frequency can be changed by control voltage Vc.
FIG. 2 equivalently shows only capacitance components viewed from quartz-crystal element 2 extracted from the aforementioned voltage-controlled oscillator. In FIG. 2, CTr represents the capacitance component of transistor 1, while CD1 and CD2 represent the capacitance components of first and second variable capacitance diodes D1, D2, respectively. Specifically, when viewed from quartz-crystal element 2, capacitance component CTr of transistor 1 is connected in parallel with a circuit including capacitances CD1, CD2 of variable capacitance diodes D1, D2 and capacitors C1, C2 for DC blocking connected in series. In other words, capacitors C1, C2 are inserted in series into a resonant loop comprising quartz-crystal element 2 and variable capacitance diodes D1, D2. In such a circuit, since a change in capacitance of first and second variable capacitance diodes D1, D2 caused by control voltage Vc is canceled with capacitors C1, C2 and reduced as a whole, it is difficult to achieve a large amount of change in the oscillating frequency.
A large amount of change in the oscillating frequency can be achieved by using capacitors C1, C2 for DC blocking with large capacitance values. In this case, however, an IC chip including the voltage-controlled oscillator inevitably has a larger size, thereby presenting a problem of impairing a reduction in size.
It is an object of the present invention to provide a voltage-controlled oscillator using a transistor as an element for oscillation and amplification, allowing a reduction in size when it is formed as an IC chip, and achieving a large amount of change in the oscillating frequency.
The present inventors focused attention on xe2x80x9cVoltage-Controlled Inverter Oscillator Circuitxe2x80x9d disclosed in Japanese Patent Laid-open Publication No. 7-273547 (JP, A, 07273547) by the assignee of the present application. The inverter oscillator circuit is a voltage-controlled type oscillator circuit using an inverter serving as a logical gate, and prevents a reduction in effectual capacitance change by connecting a DC blocking capacitor to a variable capacitance diode in parallel, rather than in series. The present inventors examined the application of the technique disclosed in the publication to a voltage-controlled oscillator including a transistor having an emitter grounded, and a base and a collector between which a quartz-crystal element is connected, and have completed the present invention.
The voltage-controlled oscillator of the present invention comprises a transistor for oscillation and amplification, first and second nodes, a piezoelectric vibrator such as a quartz-crystal element inserted between the first and second nodes, a first capacitor connecting the first node with a first terminal of the transistor, a second capacitor connecting the second node with a second terminal of the transistor, a first variable capacitance diode connected to the first node, a second variable capacitance diode connected to the second node, and means for applying a control voltage to the first and second variable capacitance diodes.
In the present invention, with the aforementioned configuration, the first and second capacitors for DC blocking are equivalently connected in parallel with the first and second variable capacitance diodes. Thus, an effectual capacitance change in the first and second variable capacitance diodes is not reduced. The configuration allows the use of the first and second capacitors with small capacitance values, a reduction in size of an IC chip when the voltage-controlled oscillator is formed as the IC chip, and a wide changing range of the oscillating frequency.